1. Technical Field of the Invention
The present invention relates to a rear-retaining structure for dynamic hip screw (DHS). Especially, the present invention relates to a rear-retaining structure attached to a dynamic hip screw to stop the slip back or provide a limited macro-slip back of the lag screw and the compression screw of DHS.
2. Prior Arts
Clinically, proximal femoral fractures are common hip fractures. Internal fixation systems, such as DHS system and intramedullary fixation system are used in the current treatments. According to clinical implementations and researches, the therapeutic effect of DHS on the stable proximal femoral fracture has been affirmed.
As showed in FIG. 1, a typical DHS system comprises: a lag screw 1 to be inserted into the neck of the femur to fix the proximal end of a fracture; a sideplate 2 connected with a sleeve 3 set at the end of the lag screw 1 to fix the distal end of the fracture, the sleeve 3 being jacketed outside the lag screw 1 to support the lag screw 1 and prevent the rotation of the lag screw 1; and a compression screw 4 is locked into the lag screw 1 from the distal end of the sleeve 3. The tightening of the compression screw 4 will cause the lag screw 1 to lead the proximal end of the fracture outward and downward along the axis of the sleeve 3, because the supporting function of the sleeve 3. As a result, the ends of the fracture may be abutted to each other and pressed forces can be controlled to strengthen the component force perpendicular to the fracture line, thereby promoting the healing of the fracture.
In the healing process of proximal femur fracture, there are some factors causing a lot of stress to the lag screw 1 and compression screw 4 of DHS, including the component force for neck inversion, weight pressure, shear forces from muscle stretches, external rotation force of lower limb, cyclical stress-strain changes occurring at the fracture, and axial micro-movement bearing at the ends of fracture. The stress may cause the lag screw 1 and compression screw 4 to produce an excessive sliding back phenomenon (referred to as “slip back”), as showed in FIG. 2. The back-out of the lag screw 1 and compression screw 4 will lead to the shortening of bone torque (D1) and bone length (D2), which makes the length of the femur shorter after healing, and as a result the patients appear limp.
In some of the fracture healing process, allowing the lag screw 1 and compression screw 4 a limited sliding back, referred to as “macro slip back”, will reduce the shear and stress concentration at the fracture, and prevent the downward shift of proximal fracture bone and the formation of collapse, inversion and slumping of femoral neck angle. But in other fracture healing process, it is also beneficial for fracture healing to fix the lag screw 1 and compression screw 4 for blocking their slip back. Therefore, whether the lag screw 1 and compression screw 4 should be in fixation or macro slip back depends on the individual condition of fractures.